Internet Engineering Task Force (IETF) S. Saxena, Ed.
Request for Comments: 6425 G. Swallow
Updates: 4379 Z. Ali
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 A. Farrel
Juniper Networks
S. Yasukawa
NTT Corporation
T. Nadeau
CA Technologies
November 2011
Detecting Data-Plane Failures inPoint-to-Multipoint MPLS - Extensions to LSP Ping
Abstract
Recent proposals have extended the scope of Multiprotocol Label
Switching (MPLS) Label Switched Paths (LSPs) to encompass point-to-
multipoint (P2MP) LSPs.
The requirement for a simple and efficient mechanism that can be used
to detect data-plane failures in point-to-point (P2P) MPLS LSPs has
been recognized and has led to the development of techniques for
fault detection and isolation commonly referred to as "LSP ping".
The scope of this document is fault detection and isolation for P2MP
MPLS LSPs. This documents does not replace any of the mechanisms of
LSP ping, but clarifies their applicability to MPLS P2MP LSPs, and
extends the techniques and mechanisms of LSP ping to the MPLS P2MP
environment.
This document updates RFC 4379.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6425.
Saxena, et al. Standards Track [Page 1]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
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Table of Contents
1. Introduction ....................................................31.1. Design Considerations ......................................41.2. Terminology ................................................42. Notes on Motivation .............................................52.1. Basic Motivations for LSP Ping .............................52.2. Motivations for LSP Ping for P2MP LSPs .....................63. Packet Format ...................................................73.1. Identifying the LSP Under Test .............................83.1.1. Identifying a P2MP MPLS TE LSP ......................83.1.1.1. RSVP P2MP IPv4 Session Sub-TLV .............83.1.1.2. RSVP P2MP IPv6 Session Sub-TLV .............93.1.2. Identifying a Multicast LDP LSP .....................93.1.2.1. Multicast LDP FEC Stack Sub-TLVs ..........10
3.1.2.2. Applicability to
Multipoint-to-Multipoint LSPs .............113.2. Limiting the Scope of Responses ...........................113.2.1. Egress Address P2MP Responder Sub-TLVs .............123.2.2. Node Address P2MP Responder Sub-TLVs ...............133.3. Preventing Congestion of Echo Replies .....................14Saxena, et al. Standards Track [Page 2]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
The requirements for P2MP extensions to the Label Distribution
Protocol (LDP) are stated in [RFC6348]. [RFC6388] specifies
extensions to LDP for P2MP MPLS.
P2MP MPLS LSPs are at least as vulnerable to data-plane faults or to
discrepancies between the control and data planes as their P2P
counterparts. Therefore, mechanisms are needed to detect such data
plane faults in P2MP MPLS LSPs as described in [RFC4687].
This document extends the techniques described in [RFC4379] such that
they may be applied to P2MP MPLS LSPs. This document stresses the
reuse of existing LSP ping mechanisms used for P2P LSPs, and applies
them to P2MP MPLS LSPs in order to simplify implementation and
network operation.
1.1. Design Considerations
An important consideration for designing LSP ping for P2MP MPLS LSPs
is that every attempt is made to use or extend existing mechanisms
rather than invent new mechanisms.
As for P2P LSPs, a critical requirement is that the echo request
messages follow the same data path that normal MPLS packets traverse.
However, as can be seen, this notion needs to be extended for P2MP
MPLS LSPs, as in this case an MPLS packet is replicated so that it
arrives at each egress (or leaf) of the P2MP tree.
MPLS echo requests are meant primarily to validate the data plane,
and they can then be used to validate data-plane state against the
control plane. They may also be used to bootstrap other Operations,
Administration, and Maintenance (OAM) procedures such as [RFC5884].
As pointed out in [RFC4379], mechanisms to check the liveness,
function, and consistency of the control plane are valuable, but such
mechanisms are not a feature of LSP ping and are not covered in this
document.
As is described in [RFC4379], to avoid potential denial-of-service
attacks, it is RECOMMENDED to regulate the LSP ping traffic passed to
the control plane. A rate limiter should be applied to the incoming
LSP ping traffic.
1.2. Terminology
The terminology used in this document for P2MP MPLS can be found in
[RFC4461]. The terminology for MPLS OAM can be found in [RFC4379].
In particular, the notation <RSC> refers to the Return Subcode as
defined in Section 3.1. of [RFC4379].
Saxena, et al. Standards Track [Page 4]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Notes on Motivation2.1. Basic Motivations for LSP Ping
The motivations listed in [RFC4379] are reproduced here for
completeness.
When an LSP fails to deliver user traffic, the failure cannot
always be detected by the MPLS control plane. There is a need to
provide a tool that enables users to detect such traffic "black
holes" or misrouting within a reasonable period of time. A
mechanism to isolate faults is also required.
[RFC4379] describes a mechanism that accomplishes these goals.
This mechanism is modeled after the ping/traceroute paradigm: ping
(ICMP echo request [RFC792]) is used for connectivity checks, and
traceroute is used for hop-by-hop fault localization as well as
path tracing. [RFC4379] specifies a "ping mode" and a
"traceroute" mode for testing MPLS LSPs.
The basic idea as expressed in [RFC4379] is to test that the
packets that belong to a particular Forwarding Equivalence Class
(FEC) actually end their MPLS path on an LSR that is an egress for
that FEC. [RFC4379] achieves this test by sending a packet
(called an "MPLS echo request") along the same data path as other
packets belonging to this FEC. An MPLS echo request also carries
information about the FEC whose MPLS path is being verified. This
echo request is forwarded just like any other packet belonging to
that FEC. In "ping" mode (basic connectivity check), the packet
should reach the end of the path, at which point it is sent to the
control plane of the egress LSR, which then verifies that it is
indeed an egress for the FEC. In "traceroute" mode (fault
isolation), the packet is sent to the control plane of each
transit LSR, which performs various checks that it is indeed a
transit LSR for this path; this LSR also returns further
information that helps to check the control plane against the data
plane, i.e., that forwarding matches what the routing protocols
determined as the path.
One way these tools can be used is to periodically ping a FEC to
ensure connectivity. If the ping fails, one can then initiate a
traceroute to determine where the fault lies. One can also
Saxena, et al. Standards Track [Page 5]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
periodically traceroute FECs to verify that forwarding matches the
control plane; however, this places a greater burden on transit
LSRs and should be used with caution.
2.2. Motivations for LSP Ping for P2MP LSPs
As stated in [RFC4687], MPLS has been extended to encompass P2MP
LSPs. As with P2P MPLS LSPs, the requirement to detect, handle, and
diagnose control- and data-plane defects is critical. For operators
deploying services based on P2MP MPLS LSPs, the detection and
specification of how to handle those defects is important because
such defects may affect the fundamentals of an MPLS network, but also
because they may impact service-level-specification commitments for
customers of their network.
P2MP LDP [RFC6388] uses LDP to establish multicast LSPs. These LSPs
distribute data from a single source to one or more destinations
across the network according to the next hops indicated by the
routing protocols. Each LSP is identified by an MPLS multicast FEC.
P2MP MPLS TE LSPs [RFC4875] may be viewed as MPLS tunnels with a
single ingress and multiple egresses. The tunnels, built on P2MP
LSPs, are explicitly routed through the network. There is no concept
or applicability of a FEC in the context of a P2MP MPLS TE LSP.
MPLS packets inserted at the ingress of a P2MP LSP are delivered
equally (barring faults) to all egresses. In consequence, the basic
idea of LSP ping for P2MP MPLS LSPs may be expressed as an intention
to test that packets that enter (at the ingress) a particular P2MP
LSP actually end their MPLS path on the LSRs that are the (intended)
egresses for that LSP. The idea may be extended to check selectively
that such packets reach specific egresses.
The technique in this document makes this test by sending an LSP ping
echo request message along the same data path as the MPLS packets.
An echo request also carries the identification of the P2MP MPLS LSP
(multicast LSP or P2MP TE LSP) that it is testing. The echo request
is forwarded just as any other packet using that LSP, and so is
replicated at branch points of the LSP and should be delivered to all
egresses.
In "ping" mode (basic connectivity check), the echo request should
reach the end of the path, at which point it is sent to the control
plane of the egress LSRs, which verify that they are indeed an egress
(leaf) of the P2MP LSP. An echo reply message is sent by an egress
to the ingress to confirm the successful receipt (or announce the
erroneous arrival) of the echo request.
Saxena, et al. Standards Track [Page 6]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
In "traceroute" mode (fault isolation), the echo request is sent to
the control plane at each transit LSR, and the control plane checks
that it is indeed a transit LSR for this P2MP MPLS LSP. The transit
LSR returns information about the outgoing paths. This information
can be used by ingress LSRs to build topology or by downstream LSRs
to do extra label verification.
P2MP MPLS LSPs may have many egresses, and it is not necessarily the
intention of the initiator of the ping or traceroute operation to
collect information about the connectivity or path to all egresses.
Indeed, in the event of pinging all egresses of a large P2MP MPLS
LSP, it might be expected that a large number of echo replies would
arrive at the ingress independently but at approximately the same
time. Under some circumstances this might cause congestion at or
around the ingress LSR. The procedures described in this document
provide two mechanisms to control echo replies.
The first procedure allows the responders to randomly delay (or
jitter) their replies so that the chances of swamping the ingress are
reduced. The second procedure allows the initiator to limit the
scope of an LSP ping echo request (ping or traceroute mode) to one
specific intended egress.
LSP ping can be used to periodically ping a P2MP MPLS LSP to ensure
connectivity to any or all of the egresses. If the ping fails, the
operator or an automated process can then initiate a traceroute to
determine where the fault is located within the network. A
traceroute may also be used periodically to verify that data-plane
forwarding matches the control-plane state; however, this places an
increased burden on transit LSRs and should be used infrequently and
with caution.
3. Packet Format
The basic structure of the LSP ping packet remains the same as
described in [RFC4379]. Some new TLVs and sub-TLVs are required to
support the new functionality. They are described in the following
sections.
Saxena, et al. Standards Track [Page 7]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
Opaque Value
An opaque value element that uniquely identifies the P2MP LSP in
the context of the Root LSR.
If the Address Family is IPv4, the Address Length MUST be 4. If the
Address Family is IPv6, the Address Length MUST be 16. No other
Address Family values are defined at present.
3.1.2.2. Applicability to Multipoint-to-Multipoint LSPs
The mechanisms defined in this document can be extended to include
Multipoint-to-Multipoint (MP2MP) Multicast LSPs. In an MP2MP LSP
tree, any leaf node can be treated like a head node of a P2MP tree.
In other words, for MPLS OAM purposes, the MP2MP tree can be treated
like a collection of P2MP trees, with each MP2MP leaf node acting
like a P2MP head-end node. When a leaf node is acting like a P2MP
head-end node, the remaining leaf nodes act like egress or bud nodes.
3.2. Limiting the Scope of Responses
A new TLV is defined for inclusion in the echo request message.
The P2MP Responder Identifier TLV is assigned the TLV type value 11
and is encoded as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type = 11 (P2MP Responder ID)| Length = Variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Sub-TLVs ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-TLVs:
Zero, one, or more sub-TLVs as defined below.
If no sub-TLVs are present, the TLV MUST be processed as if it
were absent. If more than one sub-TLV is present, the first
TLV MUST be processed as described in this document, and
subsequent sub-TLVs SHOULD be ignored. Interpretation of
additional sub-TLVs may be defined in future documents.
The P2MP Responder Identifier TLV only has meaning on an echo request
message. If present on an echo reply message, it MUST be ignored.
Saxena, et al. Standards Track [Page 11]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
in the sub-TLV does not match any address that is local to the node.
The address in the sub-TLV may be of any physical interface or may be
the router ID of the node itself.
The address in this sub-TLV SHOULD be of any transit, branch, bud, or
egress node for that P2MP LSP. The address of a node that is not on
the P2MP LSP MAY be used as a check for that no reply is received.
3.3. Preventing Congestion of Echo Replies
A new TLV is defined for inclusion in the Echo request message.
The Echo Jitter TLV is assigned the TLV type value 12 and is encoded
as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 12 (Jitter TLV) | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Jitter Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Jitter Time:
This field specifies the upper bound of the jitter period that
should be applied by a responding node to determine how long to
wait before sending an echo reply. A responding node MUST wait
a random amount of time between zero milliseconds and the value
specified in this field.
Jitter time is specified in milliseconds.
The Echo Jitter TLV only has meaning on an echo request message. If
present on an echo reply message, it MUST be ignored.
3.4. Respond Only If TTL Expired Flag
A new flag is being introduced in the Global Flags field defined in
[RFC4379]. The new format of the Global Flags field is:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ |T|V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The V flag is described in [RFC4379].
Saxena, et al. Standards Track [Page 14]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
The T (Respond Only If TTL Expired) flag MUST be set only in the echo
request packet by the sender. This flag MUST NOT be set in the echo
reply packet. If this flag is set in an echo reply packet, then it
MUST be ignored.
If the T flag is set to 0, then the receiving node MUST process the
incoming echo request.
If the T flag is set to 1 and the TTL of the incoming MPLS label is
equal to 1, then the receiving node MUST process the incoming echo
request.
If the T flag is set to 1 and the TTL of the incoming MPLS label is
more than 1, then the receiving node MUST drop the incoming echo
request and MUST NOT send any echo reply to the sender.
If the T flag is set to 1 and there are no incoming MPLS labels in
the echo request packet, then a bud node with PHP configured MAY
choose to not respond to this echo request. All other nodes MUST
ignore this bit and respond as per regular processing.
3.5. Downstream Detailed Mapping TLV
The Downstream Detailed Mapping TLV is described in [RFC6424]. A
transit, branch or bud node can use the Downstream Detailed Mapping
TLV to return multiple Return Codes for different downstream paths.
This functionality can not be achieved via the Downstream Mapping
TLV. As per Section 3.4 of [RFC6424], the Downstream Mapping TLV as
described in [RFC4379] is being deprecated.
Therefore, for P2MP, a node MUST support the Downstream Detailed
Mapping TLV. The Downstream Mapping TLV [RFC4379] is not appropriate
for P2MP traceroute functionality and MUST NOT be included in an Echo
Request message. When responding to an RSVP IPv4/IPv6 P2MP Session
FEC type or a Multicast P2MP/MP2MP LDP FEC type, a node MUST ignore
any Downstream Mapping TLV it receives in the echo request and MUST
continue processing as if the Downstream Mapping TLV is not present.
The details of the Return Codes to be used in the Downstream Detailed
Mapping TLV are provided in Section 4.
4. Operation of LSP Ping for a P2MP LSP
This section describes how LSP ping is applied to P2MP MPLS LSPs. As
mentioned previously, an important design consideration has been to
extend the existing LSP ping mechanism in [RFC4379] rather than
invent new mechanisms.
Saxena, et al. Standards Track [Page 15]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
As specified in [RFC4379], MPLS LSPs can be tested via a "ping" mode
or a "traceroute" mode. The ping mode is also known as "connectivity
verification" and traceroute mode is also known as "fault isolation".
Further details can be obtained from [RFC4379].
This section specifies processing of echo requests for both ping and
traceroute mode at various nodes (ingress, transit, etc.) of the P2MP
LSP.
4.1. Initiating LSR Operations
The LSR initiating the echo request will follow the procedures in
[RFC4379]. The echo request will contain a Target FEC Stack TLV. To
identify the P2MP LSP under test, this TLV will contain one of the
new sub-TLVs defined in Section 3.1. Additionally, there may be
other optional TLVs present.
4.1.1. Limiting Responses to Echo Requests
As described in Section 2.2, it may be desirable to restrict the
operation of P2MP ping or traceroute to a single egress. Since echo
requests are forwarded through the data plane without interception by
the control plane, there is no facility to limit the propagation of
echo requests, and they will automatically be forwarded to all
reachable egresses.
However, a single egress may be identified by the inclusion of a P2MP
Responder Identifier TLV. The details of this TLV and its sub-TLVs
are in Section 3.2. There are two main types of sub-TLVs in the P2MP
Responder Identifier TLV: Node Address sub-TLV and Egress Address
sub-TLV.
These sub-TLVs limit the replies either to the specified LSR only or
to any LSR on the path to the specified LSR. The former capability
is generally useful for ping mode, while the latter is more suited to
traceroute mode. An initiating LSR may indicate that it wishes all
egresses to respond to an echo request by omitting the P2MP Responder
Identifier TLV.
4.1.2. Jittered Responses to Echo Requests
The initiating LSR MAY request that the responding LSRs introduce a
random delay (or jitter) before sending the reply. The randomness of
the delay allows the replies from multiple egresses to be spread over
a time period. Thus, this technique is particularly relevant when
the entire P2MP LSP is being pinged or traced since it helps prevent
the initiating (or nearby) LSRs from being swamped by replies, or
from discarding replies due to rate limits that have been applied.
Saxena, et al. Standards Track [Page 16]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
It is desirable for the initiating LSR to be able to control the
bounds of the jitter. If the tree size is small, only a small amount
of jitter is required, but if the tree is large, greater jitter is
needed.
The initiating LSR can supply the desired value of the jitter in the
Echo Jitter TLV as defined in Section 3.3. If this TLV is present,
the responding LSR MUST delay sending a reply for a random amount of
time between zero milliseconds and the value indicated in the TLV.
If the TLV is absent, the responding egress SHOULD NOT introduce any
additional delay in responding to the echo request, but MAY delay
according to local policy.
LSP ping MUST NOT be used to attempt to measure the round-trip time
for data delivery. This is because the P2MP LSPs are unidirectional,
and the echo reply is often sent back through the control plane. The
timestamp fields in the echo request and echo reply packets MAY be
used to deduce some information about delivery times, for example the
variance in delivery times.
The use of echo jittering does not change the processes for gaining
information, but note that the responding node MUST set the value in
the Timestamp Received fields before applying any delay.
Echo reply jittering SHOULD be used for P2MP LSPs, although it MAY be
omitted for simple P2MP LSPs or when the Node Address P2MP Responder
sub-TLVs are used. If the Echo Jitter TLV is present in an echo
request for any other type of LSPs, the responding egress MAY apply
the jitter behavior as described here.
4.2. Responding LSR Operations
Usually the echo request packet will reach the egress and bud nodes.
In case of TTL Expiry, i.e., traceroute mode, the echo request packet
may stop at branch or transit nodes. In both scenarios, the echo
request will be passed on to the control plane for reply processing.
The operations at the receiving node are an extension to the existing
processing as specified in [RFC4379]. As described in that document,
a responding LSR SHOULD rate-limit the receipt of echo request
messages. After rate-limiting, the responding LSR must verify the
general sanity of the packet. If the packet is malformed or certain
TLVs are not understood, the [RFC4379] procedures must be followed
for echo reply. Similarly, the Reply Mode field determines if the
reply is required or not (and the mechanism to send it back).
Saxena, et al. Standards Track [Page 17]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
For P2MP LSP ping and traceroute, i.e., if the echo request is
carrying an RSVP P2MP FEC or a Multicast LDP FEC, the responding LSR
MUST determine whether it is part of the P2MP LSP in question by
checking with the control plane.
- If the node is not part of the P2MP LSP, it MUST respond
according to [RFC4379] processing rules.
- If the node is part of the P2MP LSP, the node must check
whether or not the echo request is directed to it.
- If a P2MP Responder Identifier TLV is present, then the node
must follow the procedures defined in Section 3.2 to
determine whether or not it should respond to the request.
The presence of a P2MP Responder Identifier TLV or a
Downstream Detailed Mapping TLV might affect the Return
Code. This is discussed in more detail later.
- If the P2MP Responder Identifier TLV is not present (or, in
the error case, is present, but does not contain any sub-
TLVs), then the node MUST respond according to [RFC4379]
processing rules.
4.2.1. Echo Reply Reporting
Echo reply messages carry Return Codes and Subcodes to indicate the
result of the LSP ping (when the ping mode is being used) as
described in [RFC4379].
When the responding node reports that it is an egress, it is clear
that the echo reply applies only to that reporting node. Similarly,
when a node reports that it does not form part of the LSP described
by the FEC, then it is clear that the echo reply applies only to that
reporting node. However, an echo reply message that reports an error
from a transit node may apply to multiple egress nodes (i.e., leaves)
downstream of the reporting node. In the case of the ping mode of
operation, it is not possible to correlate the reporting node to the
affected egresses unless the topology of the P2MP tree is already
known, and it may be necessary to use the traceroute mode of
operation to further diagnose the LSP.
Note that a transit node may discover an error, but it may also
determine that while it does lie on the path of the LSP under test,
it does not lie on the path to the specific egress being tested. In
this case, the node SHOULD NOT generate an echo reply unless there is
a specific error condition that needs to be communicated.
Saxena, et al. Standards Track [Page 18]

RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
The following sections describe the expected values of Return Codes
for various nodes in a P2MP LSP. It is assumed that the sanity and
other checks have been performed and an echo reply is being sent
back. As mentioned in Section 4.2, the Return Code might change
based on the presence of a Responder Identifier TLV or Downstream
Detailed Mapping TLV.
4.2.1.1. Responses from Transit and Branch Nodes
The presence of a Responder Identifier TLV does not influence the
choice of the Return Code. For a success response, the Return Code
MAY be set to value 8 ('Label switched at stack-depth <RSC>'). The
notation <RSC> refers to the Return Subcode as defined in Section3.1. of [RFC4379]. For error conditions, use appropriate values
defined in [RFC4379].
The presence of a Downstream Detailed Mapping TLV will influence the
choice of Return Code. As per [RFC6424], the Return Code in the echo
reply header MAY be set to 'See DDM TLV for Return Code and Return
Subcode' as defined in [RFC6424]. The Return Code for each
Downstream Detailed Mapping TLV will depend on the downstream path as
described in [RFC6424].
There will be a Downstream Detailed Mapping TLV for each downstream
path being reported in the echo reply. Hence, for transit nodes,
there will be only one such TLV, and for branch nodes, there will be
more than one. If there is an Egress Address Responder sub-TLV, then
the branch node will include only one Downstream Detailed Mapping TLV
corresponding to the downstream path required to reach the address
specified in the Egress Address sub-TLV.
4.2.1.2. Responses from Egress Nodes
The presence of a Responder Identifier TLV does not influence the
choice of the Return Code. For a success response, the Return Code
MAY be set to value 3 ('Replying router is an egress for the FEC at
stack-depth <RSC>'). For error conditions, use appropriate values
defined in [RFC4379].
The presence of the Downstream Detailed Mapping TLV does not
influence the choice of Return Code. Egress nodes do not put in any
Downstream Detailed Mapping TLV in the echo reply [RFC6424].
4.2.1.3. Responses from Bud Nodes
The case of bud nodes is more complex than other types of nodes. The
node might behave as either an egress node or a transit node, or a
combination of an egress and branch node. This behavior is
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RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
determined by the presence of any Responder Identifier TLV and the
type of sub-TLV in it. Similarly, the Downstream Detailed Mapping
TLV can influence the Return Code values.
To determine the behavior of the bud node, use the following rules.
The intent of these rules is to figure out if the echo request is
meant for all nodes, or just this node, or for another node reachable
through this node or for a different section of the tree. In the
first case, the node will behave like a combination of egress and
branch node; in the second case, the node will behave like pure
egress node; in the third case, the node will behave like a transit
node; and in the last case, no reply will be sent back.
Node behavior rules:
- If the Responder Identifier TLV is not present, then the node
will behave as a combination of egress and branch node.
- If the Responder Identifier TLV containing a Node Address sub-
TLV is present, and:
- If the address specified in the sub-TLV matches to an
address in the node, then the node will behave like a
combination of egress and branch node.
- If the address specified in the sub-TLV does not match any
address in the node, then no reply will be sent.
- If the Responder Identifier TLV containing an Egress Address
sub-TLV is present, and:
- If the address specified in the sub-TLV matches to an
address in the node, then the node will behave like an
egress node only.
- If the node lies on the path to the address specified in the
sub-TLV, then the node will behave like a transit node.
- If the node does not lie on the path to the address
specified in the sub-TLV, then no reply will be sent.
Once the node behavior has been determined, the possible values for
Return Codes are as follows:
- If the node is behaving as an egress node only, then for a
success response, the Return Code MAY be set to value 3
('Replying router is an egress for the FEC at stack-depth
<RSC>'). For error conditions, use appropriate values defined
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RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
in [RFC4379]. The echo reply MUST NOT contain any Downstream
Detailed Mapping TLV, even if one is present in the echo
request.
- If the node is behaving as a transit node, and:
- If a Downstream Detailed Mapping TLV is not present, then
for a success response, the Return Code MAY be set to value
8 ('Label switched at stack-depth <RSC>'). For error
conditions, use appropriate values defined in [RFC4379].
- If a Downstream Detailed Mapping TLV is present, then the
Return Code MAY be set to 'See DDM TLV for Return Code and
Return Subcode' as defined in [RFC6424]. The Return Code
for the Downstream Detailed Mapping TLV will depend on the
downstream path as described in [RFC6424]. There will be
only one Downstream Detailed Mapping corresponding to the
downstream path to the address specified in the Egress
Address sub-TLV.
- If the node is behaving as a combination of egress and branch
node, and:
- If a Downstream Detailed Mapping TLV is not present, then
for a success response, the Return Code MAY be set to value
3 ('Replying router is an egress for the FEC at stack-depth
<RSC>'). For error conditions, use appropriate values
defined in [RFC4379].
- If a Downstream Detailed Mapping TLV is present, then for a
success response, the Return Code MAY be set to value 3
('Replying router is an egress for the FEC at stack-depth
<RSC>'). For error conditions, use appropriate values
defined in [RFC4379]. The Return Code for the each
Downstream Detailed Mapping TLV will depend on the
downstream path as described in [RFC6424]. There will be a
Downstream Detailed Mapping for each downstream path from
the node.
4.3. Special Considerations for Traceroute4.3.1. End of Processing for Traceroutes
As specified in [RFC4379], the traceroute mode operates by sending a
series of echo requests with sequentially increasing TTL values. For
regular P2P targets, this processing stops when a valid reply is
received from the intended egress or when some errored return code is
received.
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RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
For P2MP targets, there may not be an easy way to figure out the end
of the traceroute processing, as there are multiple egress nodes.
Receiving a valid reply from an egress will not signal the end of
processing.
For P2MP TE LSP, the initiating LSR has a priori knowledge about the
number of egress nodes and their addresses. Hence, it is possible to
continue processing until a valid reply has been received from each
end point, provided that the replies can be matched correctly to the
egress nodes.
However, for Multicast LDP LSP, the initiating LSR might not always
know about all of the egress nodes. Hence, there might not be a
definitive way to estimate the end of processing for traceroute.
Therefore, it is RECOMMENDED that traceroute operations provide for a
configurable upper limit on TTL values. Hence, the user can choose
the depth to which the tree will be probed.
4.3.2. Multiple Responses from Bud and Egress Nodes
The P2MP traceroute may continue even after it has received a valid
reply from a bud or egress node, as there may be more nodes at deeper
levels. Hence, for subsequent TTL values, a bud or egress node that
has previously replied would continue to get new echo requests.
Since each echo request is handled independently from previous
requests, these bud and egress nodes will keep on responding to the
traceroute echo requests. This can cause an extra processing burden
for the initiating LSR and these bud or egress LSRs.
To prevent a bud or egress node from sending multiple replies in the
same traceroute operation, a new "Respond Only If TTL Expired" flag
is being introduced. This flag is described in Section 3.4.
It is RECOMMENDED that this flag be used for P2MP traceroute mode
only. By using this flag, extraneous replies from bud and egress
nodes can be reduced. If PHP is being used in the P2MP tree, then
bud and egress nodes will not get any labels with the echo request
packet. Hence, this mechanism will not be effective for PHP
scenarios.
4.3.3. Non-Response to Traceroute Echo Requests
There are multiple reasons for which an ingress node may not receive
a reply to its echo request. For example, the transit node has
failed or the transit node does not support LSP ping.
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RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
When no reply to an echo request is received by the ingress, then (as
per [RFC4379]) the subsequent echo request with a larger TTL SHOULD
be sent in order to trace further toward the egress, although the
ingress MAY halt the procedure at this point. The time that an
ingress waits before sending the subsequent echo request is an
implementation choice.
4.3.4. Use of Downstream Detailed Mapping TLV in Echo Requests
As described in Section 4.6 of [RFC4379], an initiating LSR, during
traceroute, SHOULD copy the Downstream Mapping(s) into its next echo
request(s). However, for P2MP LSPs, the initiating LSR will receive
multiple sets of Downstream Detailed Mapping TLVs from different
nodes. It is not practical to copy all of them into the next echo
request. Hence, this behavior is being modified for P2MP LSPs. If
the echo request is destined for more than one node, then the
Downstream IP Address field of the Downstream Detailed Mapping TLV
MUST be set to the ALLROUTERS multicast address, and the Address Type
field MUST be set to either IPv4 Unnumbered or IPv6 Unnumbered
depending on the Target FEC Stack TLV.
If an Egress Address Responder sub-TLV is being used, then the
traceroute is limited to only one egress. Therefore this traceroute
is effectively behaving like a P2P traceroute. In this scenario, as
per Section 4.2, the echo replies from intermediate nodes will
contain only one Downstream Detailed Mapping TLV corresponding to the
downstream path required to reach the address specified in the Egress
Address sub-TLV. For this case, the echo request packet MAY reuse a
received Downstream Detailed Mapping TLV. This will allow interface
validation to be performed as per [RFC4379].
4.3.5. Cross-Over Node Processing
A cross-over node will require slightly different processing for
traceroute mode. The following definition of cross-over is taken
from [RFC4875].
The term "cross-over" refers to the case of an ingress or transit
node that creates a branch of a P2MP LSP, a cross-over branch,
that intersects the P2MP LSP at another node farther down the
tree. It is unlike re-merge in that, at the intersecting node,
the cross-over branch has a different outgoing interface as well
as a different incoming interface.
During traceroute, a cross-over node will receive the echo requests
via each of its input interfaces. Therefore, the Downstream Detailed
Mapping TLV in the echo reply MUST carry information only about the
outgoing interface corresponding to the input interface.
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RFC 6425 P2MP MPLS Extensions to LSP PING November 2011
If this restriction is applied, the cross-over node will not
duplicate the outgoing interface information in each of the echo
request it receives via the different input interfaces. This will
reflect the actual packet replication in the data plane.
5. Non-Compliant Routers
If a node for a P2MP LSP does not support MPLS LSP ping, then no
reply will be sent, causing an incorrect result on the initiating
LSR. There is no protection for this situation, and operators may
wish to ensure that all nodes for P2MP LSPs are all equally capable
of supporting this function.
If the non-compliant node is an egress, then the traceroute mode can
be used to verify the LSP nearly all the way to the egress, leaving
the final hop to be verified manually.
If the non-compliant node is a branch or transit node, then it should
not impact ping mode. However the node will not respond during
traceroute mode.
6. OAM and Management Considerations
The procedures in this document provide OAM functions for P2MP MPLS
LSPs and may be used to enable bootstrapping of other OAM procedures.
In order to be fully operational, several considerations apply.
- Scaling concerns dictate that only cautious use of LSP ping
should be made. In particular, sending an LSP ping to all
egresses of a P2MP MPLS LSP could result in congestion at or
near the ingress when the replies arrive.
Further, incautious use of timers to generate LSP ping echo
requests either in ping mode or especially in traceroute may
lead to significant degradation of network performance.
- Management interfaces should allow an operator full control
over the operation of LSP ping. In particular, such interfaces
should provide the ability to limit the scope of an LSP ping
echo request for a P2MP MPLS LSP to a single egress.
Such interfaces should also provide the ability to disable all
active LSP ping operations, to provide a quick escape if the
network becomes congested.
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RFC 6425 P2MP MPLS Extensions to LSP PING November 20117.3. New Global Flags Registry
IANA has created a new sub-registry of the "Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry. The sub-registry is called the "Global Flags" registry.
This registry tracks the assignment of 16 flags in the Global Flags
field of the MPLS LSP ping echo request message. The flags are
numbered from 0 (most significant bit, transmitted first) to 15.
New entries are assigned by Standards Action.
Initial entries in the registry are as follows:
Bit number | Name | Reference
------------+----------------------------+--------------
15 | V Flag | [RFC4379]
14 | T Flag | [RFC6425]
13-0 | Unassigned |
8. Security Considerations
This document does not introduce security concerns over and above
those described in [RFC4379]. Note that because of the scalability
implications of many egresses to P2MP MPLS LSPs, there is a stronger
concern about regulating the LSP ping traffic passed to the control
plane by the use of a rate limiter applied to the LSP ping well-known
UDP port. This rate limiting might lead to false indications of LSP
failure.
9. Acknowledgements
The authors would like to acknowledge the authors of [RFC4379] for
their work, which is substantially re-used in this document. Also,
thanks to the members of the MBONED working group for their review of
this material, to Daniel King and Mustapha Aissaoui for their
reviews, and to Yakov Rekhter for useful discussions.
The authors would like to thank Bill Fenner, Vanson Lim, Danny
Prairie, Reshad Rahman, Ben Niven-Jenkins, Hannes Gredler, Nitin
Bahadur, Tetsuya Murakami, Michael Hua, Michael Wildt, Dipa Thakkar,
Sam Aldrin, and IJsbrand Wijnands for their comments and suggestions.
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